"Prototype development of a special version of the Pz Kpfw IV with hydrostatic drive, which was ordered by the SS in July 1944. Placing of this drive at the rear of the tank, gave additional space within the fighting compartment, and a drawing dated October 1944, indicated that sloping frontal armour was to be a feature of the project. A prototype was build from a Pz Kpfw IV Ausf G." - Encyclopedia of german tanks of world war two, p. 115.

Long 75 prolly means the 75L/70 as found on the StuG III, Pz IV and JagdPz IV as well as a few other tanks/TD's (strictly speaking not the exact same gun between tanks and TD's but well.... basically ends up on the same characteristics).

BTW, I saw this topic before the patch notes i did not know they were actually going to put it in the game. Any details on this tank compared to the Regular Pz IV? I know NOTHING about the Hydro, what's it's difference with the regular Pz IV??

Doesn`t the stock PzIv also have a rear mounted engine? As a matter of fact, let me answer that question: yes! However, as with many tanks of that era, the gearbox was connected to drive sprockets (big cogs that make the treads move) that were located in the front. So exactly how this "new" PzIV is any different is beyond me.
I would like to take a guess at why it was developed in the first place. 'Hydrostatic' means a hydrostatic automatic gearbox. Which was slated for all German tanks at the end of the war.